Starch amorphous

Aguilera, J.M., Castro, L., and Cadoche, L. Structure relationships in a starch amorphous model, Drying — Proceedings of the 14th International Drying Symposium, Sao Paulo, Brazil, Vol. B, p. 1468, 2004. 

The qua si-crystalline stmcture of natural starch granules causes them to be insoluble in water at normal room temperature and gives them relative resistance to carbohydrases other than a-amylase and glucoamylase unless the granules become swollen. Three-dimensional arrangements of crystalline and amorphous zones in starch granules have been suggested (2). 

Nitrostarch can be prepared by dissolving starch in an excess of nitric acid and pouring this solution into an excess of sulfuric acid to precipitate NS as an amorphous powder. This method is uneconomical and hard to control. Consequently it is not used commercially... 

Although it has been found that the separated amylose component can be readily orientated to yield fiber patterns, amylopectin usually gives poor or amorphous patterns. In the granule, however, amylopectin does exhibit crystallinity, since waxy maize starch gives a diffraction pattern and other waxy starches behave similarly.193 -195 (This suggests that the branch points in the amylopectin molecule may be in the amorphous part of the granule.)... 

The sorption of water by excipients derived from cellulose and starch has been considered by numerous workers, with at least three thermodynamic states having been identified [82]. Water may be directly and tightly bound at a 1 1 stoichiometry per anhydroglucose unit, unrestricted water having properties almost equivalent to bulk water, or water having properties intermediate between these two extremes. The water sorption characteristics of potato starch and microcrystalline cellulose have been determined, and comparison of these is found in Fig. 11. While starch freely adsorbs water at essentially all relative humidity values, microcrystalline cellulose only does so at elevated humidity values. These trends have been interpreted in terms of the degree of available cellulosic hydroxy groups on the surfaces, and as a function of the amount of amorphous material present [83]. 

A second reason for the turn-over in the osmotic modulus may arise from a decrease in A2 until it becomes zero or even negative. This would be the classical situation for a phase separation. The reason why in a good solvent such a phase separation should occur has not yet been elucidated and remains to be answered by a fundamental theory. In one case the reason seems to be clear. This is that of starches where the branched amylopectin coexists with a certain fraction of the linear amylose. Amylose is well known to form no stable solution in water. In its amorphous stage it can be brought into solution, but it then quickly undergoes a liquid-solid transition. Thus in starches the amylose content makes the amylopectin solution unstable and finally causes gelation that actually is a kinetically inhibited phase transition [166]. Because of the not yet fully clarified situation this turn-over will be not discussed any further. 

Gelatinization, as we understand now, is not only associated with crystalline order, but is also influenced by structural changes in the amorphous region. XRD does not detect or account for the structural changes that occur in the amorphous regions of the starch granules. 

FIGURE 5.7 X-ray diffraction profiles of native (ungelatinized), partially gelatinized, and completely gelatinized (amorphous) tapioca starch. Reprinted from Carbohydrate Polymers, Vol. 67, Ratnayake and Jackson (2007), A new insight into the gelatinization process of native starches. Pages 511-529, 2007, with permission from Elsevier. 

Nakazawa et al. (1984) argued that when starch-water mixtures (30-50% starch) are held at a certain temperature (55-80 °C), for a certain period (0-45 h), and depending on the time-temperature combination, starch granules increase their amorphous portion and decrease their crystalline portion. These amorphous and crystalline phases melted sequentially during DSC phase transition experiments. Their experiments... 

---